There is mounting evidence that the over-production of the lysosomal enzyme, Cathepsin D, can be a useful prognostic marker in breast cancer. Cathepsin D is synthesized as a 52 kDa precursor (pro-CD)) that is processed into the active mature form made up of 34k and 14k subunits. Although pro-CD is the form that is abnormally secreted by tumor cells, currently available assays measure the mature form and thus normal levels of endogenous enzyme may confound the interpretation of test results. An assay that is specific for the 52 kDa pro-form could prove to be a more reliable prognostic indicator for breast as well as other types of cancer. The goal of this proposal is to generate monoclonal and polyclonal antibodies which specifically recognize epitopes in the pro-sequence of cathepsin D. Two peptides derived from this sequence will be synthesized and used to immunize mice and rabbits. Resulting antibodies will be used together with ones that already exist to format an ELISA for the quantitation of pro-CD. The assay will be used to evaluate a small number of sera and tumor samples from breast cancer patients.